1. Field of the Invention
This invention relates to a drive control system of an electric vehicle. More particularly, it relates to the drive control system of the electric vehicle that controls wheel slip by limiting the power to the motor.
2. Related Art
In a vehicle that runs by generating torque at the driving wheels, when a driving torque supplied to the driving wheels becomes larger than friction generated between the driving wheels and a road surface, the driving wheels start to turn without gripping, which indicates that slip has occurred, and controllability decreases considerably. Such slip of the driving wheels tends to occur especially when driving on frozen or wet road surfaces on which the friction is low, or when accelerating the vehicle by supplying excessive torque to the driving wheels.
Then, for example, in the Japanese Patent No. 2674828, an electric vehicle is disclosed in which the slip is detected electronically, and if the slip is judged to occur, the motor current is decreased to decrease the driving torque and thus the slip is prevented.
More specifically, this electric vehicle detects the number of revolutions of the motor, and calculates acceleration with this number of revolutions. On the other hand, a motor current is detected, and an expected acceleration with the number of revolutions of the motor is calculated based on this motor current that will not generate the slip between the driving wheels and the road surface. Furthermore, when acceleration with the number of revolutions of the motor exceeds the expected acceleration, it is judged that the slip is generated. When it is judged that the slip is generated, the driving torque of the motor is decreased and thus the slip is suppressed.
However, in the electric vehicle described above, when the driving torque of the motor is decreased by making the feedback of the number of revolutions of the motor or the current, by considering the delay of the motor response, it takes a certain time from the occurrence of the slip to the elimination of the slip by decreasing the torque created at the driving wheels. That is, the amount of slip at the driving wheels overshoots and hence there is a risk of decreased the controllability.